Researchers from Rutgers University in New Jersey asked 21 volunteers to undertake hand-washing experiments. They found that the temperature of the water made no significant difference to how much bacteria remained on their hands after washing.
In addition, lathering hands for just 10 seconds was enough to remove germs, the report, published in the Journal of Food Protection, says.
Those who took part in the study had high levels of a harmless strain of E. coli bacteria applied to their hands. They then washed them using cold, warm or hot water; using various quantities of soap; and washing for various lengths of time. The tests were repeated several times over a six month period.
When researchers analysed the amounts of bacteria left on hands after washing, they found that water at all three temperatures worked equally well.
Co-author Donald Schaffner said using cold or cool water to wash hands—and limiting the amount of time water is running—could have significant energy and cost savings.
However, the researchers did find that very brief hand washing, for just five seconds, did not clean hands effectively, whereas washing for 10 seconds worked just as well as washing for longer durations.
That 10 seconds, however, applies only to time spent lathering, or rubbing hands together with soap, Schaffner notes. “The time you spend turning on the tap, putting soap in your hands, and rinsing afterward, those don’t count.”
But he also points out: “If you just changed a diaper or you’ve been in the garden or you’re cutting up a raw chicken, don’t think you’re good to go after 10 seconds if you can still see or feel something on your hands. By all means, keep lathering.
“If you’re uncomfortable because the water is too hot or the water is too cold, then you’re not going to do a good job.”